Calcium regulated muscle cell contraction

[dae]

I know that when a muscle cell receives a signal to contract, the sarcoplasmic reticulum releases Ca++ (which then binds to tropomyosin allowing myosin head to walk up the actin filament) but I have been told that a Ca++ deficit will cause the muscle cell to contract and not be able to release. What piece of information am I missing to make these facts coherent?

Follow up question: is this at all related to rigor mortis?

[sdekivit]

the muscle fiber will relax when no Ca(2+) is available.

It's true, though, that a deficit in ATP will cause muscle stiffness because in the ADP-form the myosinheads cant release from the troponin, which is Ca(2+) dependant in making a bindingsite for myosin. (in rigor mortis Ca(2+) will diffuse out of the ER but no ATP is available leading to stiffness of the corpse)